Hitachi HD44780 LCD controller

The Hitachi HD44780 LCD controller is an alphanumeric dot matrixliquid crystal display (LCD) controller developed by Hitachi that was commonly used during the MCS-51 era. It was made commercially available around year 1987.[citation needed] The character set of the controller includes ASCII characters, Japanese Kana characters, and some symbols in two 28 character lines. Using an extension driver, the device can display up to 80 characters.[1]

Compatible LCD screens are manufactured in several standard configurations. Common sizes are one row of eight characters (8x1), and 16×2, 20×2 and 20×4 formats. Larger custom sizes are made with 32, 40 and 80 characters and with 1, 2, 4 or 8 lines. The most commonly manufactured larger configuration is 40x4.[2] characters, which requires two individually addressable HD44780 controllers with expansion chips as a single HD44780 chip can only address up to 80 characters. A common smaller size is 16×2, and this size is readily available as surplus stock for hobbyist and prototyping work.[original research?]

Character LCDs use a 16 contact interface, commonly using pins or card edge connections on 0.1 inch (2.54 mm) centers. Those without backlights may have only 14 pins, omitting the two pins powering the light. This interface was designed to be easily hooked up to the MCS-51 XRAM interface, using only two address pins, which allowed displaying text on LCD using simple MOVX commands, offering cost effective option for adding text display to devices.[citation needed] The pinout is as follows:

Amber backlight on a HD44780 display

Ground

VCC (+3.3 to +5V)

Contrast adjustment (VO)

Register Select (RS). RS=0: Command, RS=1: Data

Read/Write (R/W). R/W=0: Write, R/W=1: Read (This pin is optional due to the fact that most of the time you will only want to write to it and not read. Therefore, in general use, this pin will be permanently connected directly to ground.)

Clock (Enable). Falling edge triggered

Bit 0 (Not used in 4-bit operation)

Bit 1 (Not used in 4-bit operation)

Bit 2 (Not used in 4-bit operation)

Bit 3 (Not used in 4-bit operation)

Bit 4

Bit 5

Bit 6

Bit 7

Backlight Anode (+) (If applicable)

Backlight Cathode (-) (If applicable)

The nominal operating voltage for LED backlights is 5V at full brightness, with dimming at lower voltages dependent on the details such as LED color. Non-LED backlights often require higher voltages.

Selecting 4-bit or 8-bit mode requires careful selection of commands. There are two primary considerations. First, with D3-D0 unconnected, these lines will always appear low (0b0000) to the HD44780 when it is in 8-bit mode. Second, the LCD may initially be in one of three states:

(State1) 8-bit mode

(State2) 4-bit mode, waiting for the first set of 4 bits

(State3) 4-bit mode, waiting for the second set of 4 bits

State3 may occur, for example, if a prior control was aborted after sending only the first 4 bits of a command while the LCD was in 4-bit mode.

The following algorithm ensures that the LCD is in the desired mode:

Set D7-D4 to 0b0011, and toggle the enable bit.

If in State1, the LCD will see the command as 0b0011_0000, and thus remain in 8-bit mode (State1).

If in State2, the LCD will simply latch the value 0b0011 into bits 7-4 and then move to State3.

If in State3, the LCD will latch the value 0b0011 into bits 3-0, and then execute a random command based on the (unknown to us) values in bits 7-4, after which it will either be in State1 (if the unknown bits happened to be 0b0011), or State2 (if the unknown bits were anything else).

Repeat the above, setting D7-D4 to 0b0011 and toggling the enable bit again.

If in State1, the LCD will remain in 8-bit mode (State1) just as above.

If in State2, it will latch the value into bits 7-4 and move to State3, just as above.

If in State3, the LCD will latch the value into bits 3-0 just as above and execute a command. However, the command will no longer be random, but will be the 0b0011 that was latched from State2 in the previous iteration. Thus, the LCD will switch to 8-bit mode and change to State1.

The LCD is now in either State1 or State 3. Repeat the previous step one more time.

If in State1, the LCD will remain in 8-bit mode (and thus State1).

The LCD can no longer be in State2 at this point.

If in State3, the LCD will latch the value into bits 3-0 and execute a command, which will be the 0b0011 that was latched from State2 in the previous iteration, thus switching the LCD to 8-bit mode and State1.

Now that the LCD is definitely in 8-bit mode, it can be switched to 4-bit mode if desired. To do so, set D7-D4 to 0b0010 and toggle the enable bit. This will leave the LCD in 4-bit mode, configured for a single line and 5x8 fonts.

Issue any desired additional Function Set commands to specify the number of lines and the font to use, being sure to use the appropriate value for bit 4 so as to remain in the desired mode (0 for 4-bit and 1 for 8-bit).

Once in 4-bit mode, character and control data are transferred as pairs of 4-bit "nibbles" on the upper data pins, D7-D4. The four most significant bits (7-4) must be written first, followed by the four least significant bits (3-0).

The original HD44780 character generator ROM contains 208 characters in a 5×8 dot matrix, and 32 characters in a 5×10 dot matrix. More recent compatible chips are available with higher resolution, matched to displays with more pixels[citation needed].

There is a Japanese version of the ROM which includes kana characters (HD44780UA00), and a European version which includes Cyrillic and Western European characters (HD44780UA02).[4]

The 7-bit ASCII subset for the Japanese version is non-standard: it supplies a Yen symbol where the backslash character is normally found, and left and right arrow symbols in place of tilde and the rub-out character.

A limited number of custom characters can be programmed into the device in the form of a bitmap using special commands. These characters have to be written to the device each time it is switched on, as they are stored in volatile memory.